The present invention relates to a method for preventing deposition of polymer scale on the inner walls of a polymerization reactor in the polymerization of an ethylenically unsaturated monomer by providing a coating on the walls and a coating agent used therefor.
As is well known, polymers of ethylenically unsaturated monomers or, in particular, vinylic monomers are produced in a variety of polymerization processes including suspension polymerization, emulsion polymerization, solution polymerization, gas-phase polymerization and bulk polymerization. One of the most serious problems common in all of these polymerization processes is the deposition of polymer scale on the reactor walls and other surfaces coming into contact with the monomer during polymerization such as the stirrer blades and the like. When polymer scale is deposited on the reactor walls and other surfaces, several disadvantages are caused thereby such as the decrease in the yield of the polymer product so much, decrease in the cooling capacity of the reactor adversely affecting the productivity and possible degradation of the product quality due to the eventual falling off and intermixing of the polymer scale in the polymer product. In addition, the reactor walls covered with the polymer scale deposited thereon must be thoroughly cleaned before preparing for the next run of the polymerization with consumption of a great deal of time and labor while such works for polymer scale removal involve a very serious problem in the workers' health due to the toxicity of the unreacted monomer contained in the polymer scale as is a matter of public concern in recent years.
A variety of methods have of course been proposed to prevent deposition of polymer scale on to the reactor walls including, for example, a method in which the reactor walls are coated prior to polymerization with a polar organic compound, e.g. amine compounds, quinone compounds, aldehyde compounds and the like, or dyes or pigments (see, for example, Japanese Patent Publications 45-30343 and 45-30835) and a method in which the coating composition is prepared by treating a polar organic compound or a dye with a metal salt (see, for example, Japanese Patent Publication 52-24953).
These prior art methods are indeed effective in the suspension polymerization of vinylic monomers for preventing polymer scale deposition on the reactor walls but the effectiveness of these methods is greatly reduced or almost no effect can be obtained with these methods for scale prevention in the emulsion polymerization of vinylic monomers. In particular, the effectiveness of these prior art methods is dependent on the types of the monomers and, if not ineffective at all, little effect can be obtained for polymer scale prevention when the polymerization mixture contains one or more of water-soluble monomers such as acrylic, methacrylic and itaconic acids.